Want to change you need to do the same as an E2 visa holder.
If you are lucky and get a LOR then a transfer is relatively simple.
If you quit without a LOR then you will need to either leave (hand in your ARC to cancel your visa), return as a tourist and start again.

DO BE AWARE that immigration is presently denying new visa confirmation numbers to people who leave early without a LOR even if they do have a canceled status of sojourn until the contractual period has expired.

Easy to quit. Might not be possible to work in Korea till the contract expires.